Radiation-sensitive compositions are routinely used in the preparation of high-performance printing plate precursors (or imageable elements). There are primarily two ways of improving the properties of radiation-sensitive compositions and thus also improving the properties of the corresponding printing plate precursors. The first way addresses improvement of the properties of the radiation-sensitive components in the compositions (frequently negative diazo resins or photoinitiators). The other way deals with improvement of physical properties of the radiation-sensitive layers through the use of novel polymeric compounds (“binders”).
The latest developments in the field of printing plate precursors are concerned with radiation-sensitive compositions that can be imagewise exposed by means of lasers or laser diodes. This type of exposure does not require films as intermediate information carriers since lasers can be controlled by computers.
High-performance lasers or laser diodes that are used in commercially available image-setters emit light in the wave-length ranges of between 800 to 850 nm and between 1060 and 1120 nm, respectively. Therefore, printing plate precursors, or initiator systems contained therein, which are to be imagewise exposed by means of such image-setters have to be sensitive in the near IR range. Such printing plate precursors can then basically be handled under daylight conditions that significantly facilitate their production and processing.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, radiation-sensitive compositions are used wherein after an imagewise exposure the exposed areas are cured. In a developing step, only the non-exposed areas are removed from the substrate. For positive-working printing plates, radiation-sensitive compositions are used whose exposed areas dissolve faster in a given developing agent than the non-exposed areas. This process is referred to as photo-solubilization.
Negative-working imageable elements that typically require a preheating step after imagewise exposure are described for example in EP Publications 0 672 544A1 (Bauer et al.), 0 672 954A1 (Haley et al.), and 0 819 985A1 (VanDamme et al.) as well as U.S. Pat. No. 5,491,046 (DeBoer et al.). In U.S. Pat. No. 5,496,903 (Watanabe et al.) and DE 196 48 311 (Cunningham et al.), photosensitive compositions are described which in addition to a dye absorbing in the IR range comprise borate or halogenated s-triazines as co-initiators.
U.S. Pat. No. 6,245,486 (Teng) discloses radiation sensitive printing plates, including on-press developable plates. U.S. Pat. No. 6,245,481 (Teng) discloses IR-ablatable, UV-photopolymerizable two-layer compositions that require IR exposure followed by UV flood irradiation. U.S. Pat. No. 5,599,650 (Bi et al.) discloses UV imageable, negative-working, on press developable printing plates based on free radical polymerization. U.S. Pat. No. 6,071,675 (Teng) discloses printing plates that include dispersed solid particles in the imaging layer.
U.S. Pat. No. 6,309,792 (Hauck et al.) describes IR-sensitive compositions comprising a polymeric binder, a free radically polymerizable system, and a specific initiator system. The compositions of WO 00/48836 require a preheat step after the exposure for sufficient hardening of the compositions and the printing plate precursors must be developed with an aqueous developer. U.S. Pat. No. 6,864,040 (Mueller et al.) describes IR sensitive compositions containing leuco dyes and requires a preheat step after IR exposure and an aqueous development step for processing.
U.S. Pat. No. 6,582,882 (Pappas et al.) discloses graft copolymers comprising polyethylene oxide side chains that may further comprise a hydrophobic segment between the polyethylene oxide segment and the main chain, and a hydrophobic segment at the terminus of the polyethylene oxide side chains. U.S. Pat. No. 6,846,614 (Timpe et al.) discloses poly(alkylene oxide) polymers and copolymers, including block copolymers of poly(ethylene oxide) and poly(propylene oxide).
On-press developable imageable elements are also described in U.S. Pat. No. 6,242,156 (Teng), U.S. Pat. No. 6,482,571 (Teng), U.S. Pat. No. 6,541,183 (Teng), U.S. Pat. No. 6,576,401 (Teng), U.S. Pat. No. 6,884,560 (Yanake et al.), and U.S. Pat. No.6,899,994 (Huang et al.) and U.S. Patent Application Publications 2003/0143488 (Teng), 2003/0165777 (Teng), 2004/0265736 (Aoshima et al.), 2004/0229165 (Munnelly et al.), and 2005/0003285 (Hayashi et al.).
U.S. Pat. No. 7,005,234 (Hoshi et al.) describes negative-working, IR-sensitive imageable elements that are said to be developed on-press by solubilizing or dispersing the non-exposed regions of the imageable layer with water. Similarly, U.S. Patent Application Publication 2005/0263021 (Mitsamoto et al.) describes removing non-exposed regions in the imaged element with an aqueous solution.
Problem to be Solved
While the literature describes numerous imaging compositions that can be used in on-press development to provide lithographic printing plates, there is a continuing need to provide improved imageable elements and imaging methods to reduce the time and impressions required to achieve complete image development. In particular, it is desired to have imageable elements that can easily and quickly be developed on-press within a few initial printing impressions and that will provide quality impressions for “short-run jobs” (for example, less than 10,000 impressions).
Moreover, there is a need to minimize contamination of the fountain solution from the imageable layer components of imageable elements during on-press development. Such contamination may lead to a color shift of the lithographic printing ink on printed sheets, dot gain characteristics of the printing system, over-emulsification of the lithographic printing inks, toning in non-imaged areas, and clogging of the fountain solution filtration system.